Skooma

Skooma, a famous illegal drug of Tamriel, was a part of The Elder Scrolls III: Morrowind. In Morrowind, this "drug" would eventually get you addicted with side effects. In Oblivion, you would find drug dens containing skooma addicts. So far in Skyrim there have been random encounters of people who sell skooma. Drinking Skooma gives you 25% stamina boost for limited time.